Heretofore, a unidirectional prepreg has been provided which is fabricated by impregnating a flat band-like unidirectional fiber bundle with a matrix resin. The flat band-like unidirectional fiber bundle is prepared by spreading and opening a unidirectional fiber bundle which is formed by bundling filaments constituting reinforcement fiber such as carbon fiber, glass fiber, and aramid fiber. The matrix resin includes: thermosetting resins such as epoxy resins, and unsaturated polyester resins; and thermoplastic resins such as polyolefin resins, aliphatic polyamide resins, and polyethylene terephthalate resins. In order to form a high-quality unidirectional prepreg, the unidirectional fiber bundle must be opened with high accuracies. In this connection, a variety of techniques for opening the unidirectional fiber bundle have been proposed. For example, Patent Literatures 1 and 2 disclose a technique where the unidirectional fiber bundle is spread widthwise and opened by passing an air flow through the unidirectional fiber bundle while moving the unidirectional fiber bundle in a longitudinal direction. The air flow is passed through the unidirectional fiber bundle in a direction perpendicular to the movement direction of the fiber bundle.
Further, a variety of techniques for impregnating the opened band-like unidirectional fiber bundle with the resin have been proposed. Disclosed in Patent Literature 3, for example, is a technique where the opened band-like unidirectional fiber bundle is impregnated with the resin by applying pressure on the unidirectional fiber bundle overlapped with a resin film by passing them through nip rolls. Further disclosed in Patent Literature 4, for example, is a technique where the unidirectional fiber bundle is impregnated with the resin by passing the opened band-like unidirectional fiber bundle overlapped with the resin film through hot rollers.